1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 17

3. Virtual Paths with ATM
Two level hierarchy of virtual connection: 8-bit VPI and 16-bit VCI
Switches in the public network use 8-bit VPI
Corporate sites use full 24-bit address (VPI + VCI)
Much less connection-state info in switches
Virtual path: fat pipe with bundle of virtual circuits

4. Physical Layers for ATM
ATM may run over several phy media
ATM was assumed to run over SONET but both are entirely separable entities
ATM frame boundaries to be correctly identified
Successive 53-byte ATM frames in payload
SONET overhead byte points to the payload
Another way is to calculate CRC (5th byte of the cell)

5. ATM and LANs
ATM grew out of the telephone community and later used for computer communication
Significant advantage of performance and better scalability of switched over shared media
No distance limitation in ATM making it a good choice for high-performance LAN backbone
Point-to-point, long distance Gigabit Ethernet is a competing technology with ATM

6. ATM as a LAN Backbone
Different from traditional LANs; no native support for broadcast or multicast

7. How to broadcast to all nodes on an ATM LAN ?
ATM in a LAN
How to broadcast to all nodes on an ATM LAN ?
Without knowing all the addresses
Without setting up VC to all of them

8. ATM in a LAN Two solutions
Redesign protocols that consider LAN different from what ATM can provide (e.g. ATMARP)
Make ATM behave like shared media, without loosing performance advantage of switched media (e.g. LANE)
ATM address is different from a unique 48-bit MAC address

9. Shared Ethernet Emulation with LANE
All hosts think they are on the same Ethernet
Ethernet
Switch H
ATM Switch H
LANE / Ethernet
Adaptor Card H H H
Ethernet
Switch H
ATM Switch H
LANE / Ethernet
Adaptor Card H H H

10. LAN Emulation (LANE) with ATM
Transparent shared media emulation of ATM
Adds (not changes) functionality to ATM switches
Each device needs a global MAC address, as well as an ATM address to establish a VC

11. LAN Emulation (LANE) with ATM
Devices connect as LAN Emulation Clients (LEC)
LANE provides Ethernet-like interface to LECs
Similar solutions for other networks: VPNs on WANs, VLANs on large, switched Ethernets

12. ATM / LANE Protocol Layers
Higher-layer
Higher-layer
protocols
protocols
(IP, ARP, . . .)
(IP, ARP, . . .)
Ethernet-like
interface
Signalling
Signalling
+ LANE
+ LANE
AAL5
AAL5
ATM
ATM
ATM
PHY
PHY
PHY
PHY
Host
Switch
Host

13. Clients and Servers in LANE
LAN Emulation Client (LEC)
Host, bridge, router or switch
LAN Emulation Server (LES)
Maintains client’s MAC and ATM addresses
Maintains ATM address of BUS

14. Clients and Servers in LANE
LAN Emulation Configuration Server (LECS)
High-level network management when LEC starts up
Reachable by preset VC (recall known server port#)
Maintains mapping of ATM address to LANE type

15. Clients and Servers in LANE
Broadcast and Unknown Server (BUS)
Emulates broadcast and multicast; critical to LANE
Uses point-to-multipoint VC with all clients
Servers physically located in one or more devices
LECS

16. LANE Registration
Client contacts LECS on predefined VC, and sends ATM address to it
LECS returns LAN type, MTU and ATM address of LES
Client signals connection to LES, and registers MAC and ATM addresses with LES
LES returns ATM address of BUS
Client signals connection to BUS
Bus adds client to point-to-multipoint VC H3
LES
BUS
ATM Network H1 H2
LECS

17. LANE Circuit Setup
Client (H1) knows destination MAC address of receiver (H2)
Client (H1) sends 1st packet to BUS
BUS sends address resolution request to LES
LES returns ATM address to client (H1)
Client (H1) signals connection to H2 for subsequent packets H3
LES
BUS
ATM Network H1 H2
LECS

18. Switches: The Intersections

19. The Intersections Design intersection to accommodate traffic flows
Islamabad
Zero Point
Pir Wadhai
Faizabad Flyover
Rawalpindi
Saddar
Rawal Dam
Faizabad
Ayub
Park
Airport
Design intersection to accommodate traffic flows

20. Contention in Switches
Some packets destined for same output
One goes first
Others delayed or dropped
Delaying packets requires buffering
Finite capacity, some packets must still drop
At inputs
Increases/adds false contention
Sometimes necessary
At outputs
Can also exert “backpressure”

21. Output Buffering 1x6 Switch Customer service Standard check-in lines x
Mr. A waiting to claim refund of Rs.100
you
Mr. X writing complaint letter
trying to check-in
1x6 Switch

22. Input Buffering: Head-of-line Blocking
Customer service
Standard check-in lines
agents are standing by ! x
1x6 Switch
Mr. X writing complaint letter a
Mr. A waiting to claim refund of Rs.100
trying to check-in
you

23. Backpressure 1x6 Switch Customer service Standard check-in lines x a
“no more, please” a
you i
trying to check-in
1x6 Switch
propagation delay requires that switch exerts backpressure before buffer is full; thus used in networks with small propagation delay

24. Backpressure
Switch 1
Switch 2
“no more, please”
Propagation delay requires that switch 2 exert backpressure at high-water mark rather than when buffer completely full
It is thus typically only used in networks with small propagation delays (e.g., switch fabrics)

25. Switching Hardware
Multi-input multi-output device, getting packets from inputs to the outputs as fast as possible
Performance of a switch is limited by I/O bus bandwidth (each packet traverse twice)
1Gbps I/O bus can support ten T3 (45 Mbps) links, three STS-3 (155 Mbps) links, and not even one STS-12 (625 Mbps) link
Success or failure of a new protocol depend on whether it takes advantage of switch’s capabilities

26. Switching Fabric Special-purpose (switching) hardware General problem
Connect N inputs to M outputs (NxM switch)
Often N=M (bidirectional links)
Design goals
High throughput: want aggregate close to MIN (sum of inputs, sum of outputs)
Avoid contention (fabric faster than ports)
Good scalability:linear size/cost groth in N/M

27. Switching Fabric and Ports
Input
Output
port
port
Input
Output
Switch
Fabric
Avoid contention here
port
Fabric
port
Switch
fabric
Input
Output
port
port
Input
Output
port
port

28. Switch: Fabric and Ports
Fabric has a job to deliver packets to the right output
Input
Output
port
port
Input
Output
Switch
fabric
(with small internal buffering)
port
Fabric
port
Input
Output
port
port
Input
Output
port
port

29. Ports and Fabric Ports deals with the complexity of the real world
Virtual circuit management is handled in ports
Determine outpt port using forwarding tables
Input port is the first in performance bottlenecks
Header processing and handling packet to fabric

30. Buffering is required at ports
Ports and Fabric
Buffering is required at ports
Buffer management has profound impact on performance
Internal (in fabric) or output buffering is normally used
Fabric: simply move packets from inputs to outputs

31. Design Goals - Throughput
An n x m switch can provide max ideal throughput of S = S1 + S2 + ……… + Sn
Only possible if traffic at inputs is evenly distributed across all outputs
Sustained throughput higher than link speed of output is not possible

32. Design Goals - Throughput
Variable size packets affect performance
Some operations have constant overhead per packet
Switch performs differently for different sizes of packets
Packet per second (pps) rate is also important
Most switches are subject to internal contention
Determine performance under diff traffic loads

33. Design Goals - Throughput
Traffic models are important to throughput
Arrival time, output port, packet length
Extremely difficult to achieve accurate models
Traffic-modeling very successful in telephony
Designers now expect high range of throughputs
In order to handle a steady stream of 64-byte packets, a 40Gbps switch need a rate of 78M pps !!!

34. Design Goals - Scalability
Cost of hardware rises fast with increasing the number of ports n
Adding ports increases hardware & design complexity
Scalability in terms of rate of increase in cost
Design complexity determines maximum switch size
Switch designs run into problems at some maximum number of inputs and outputs

35. Switch Performance Avoid contention with buffering Good scalability
Use output buffering when possible
Apply backpressure through fabric
Input buffering with “peeking” (non-FIFO semantics) to reduce head-of-line blocking problems
Drop packets if input buffer overflows
Good scalability
O(N) ports
Port design complexity O(N) gives O(N2) for switch
Port design complexity O(1) gives O(N) for switch

36. Crossbar (“Perfect”) Switch
Problem: hardware scales as O(N2)

37. Knockout Switch: Pick L from ND 4
8-to-4 concentrator D D D D 3
2x2 random selector D D D D 2 D
delay unit
Outputs
Inputs D D D D D 1
Problem: what if more than L arrive

38. Shared Memory Switch … … Inputs Outputs Mux Buffer memory Demux W rite
Read
control
control

39. Self-Routing Fabrics Use source routing on “network” within switch
Input port attaches output port number as header
Fabric routes packet based on output port
Types
Banyan network
Batcher-Banyan network
Sunshine switch

40. Banyan Network No contention if inputs are sorted and unique
Sends 0 bit up
Sends 1 bit down
MSB
LSB
No contention if inputs are sorted and unique

41. Banyan Network
MSB
LSB
Sends 0 bit up, 1 bit down

42. Batcher (Merge Sort) Network
Routing packets through a Batcher network
Batcher-Banyan Network
Attach the two-back-to-back
Arbitrary unique permutations routed without contention

43. Batcher-Banyan Network
sends 1 bit up
sends 0 bit down
sends 0 bit up
sends 1 bit down

44. (marks overflow packets)
Sunshine Switch
(marks overflow packets)
Like a Knockout switch, except
Recirculates overflow packets i.e., when more than L arrive in one cycle